General instability of a reinforced cylindrical shell has been investigated by many authors. Theories have recently been developed by Kendrick, Kaminsky, Nash, Bodner, and Terada-Shimamoto. Experimental results are in conformity with these theories only in the case of a cylindrical shell reinforced by shallow frames and supported simply at both ends. In this paper general instability of a cylindrical shell is investigated theoretically on the basis of energy concept. The theory is applicable to the case of a shell clamped at both ends. In this case a buckling pattern ω= H (1+λ/μ cosπξ/2λ) cos n η/ R , -λ<ξ<λ, λ+μ= L /2 H (1-cosπξ _i /2μ) cos n η/ R , i =1, 2, 0<ξ₁=ξ+ L /2<μ, -μ<ξ₂=ξ- L /2<0, is assumed, where parameters λ and μ determined so as to minimize the buckling pressure. The buckling pattern so determined differs with that taken by Kaminsky. Collapsing tests were conducted with two cylindrical shells, and results are in good accordance with the present theory even in the case of a shell clamped at both ends.